Based on the gate-driver on array (GOA) technology, circuitry functions of a shift register are implemented by using TFTs, and the shift register is integrated on a pixel array substrate. GOA can be manufactured under a same production process with the pixel array substrate, and thus the production flow of the display device is simplified.
Although the GOA technology has the above advantages, the GOA technology currently still has the following problems: the shift register integrated on the array substrate is highly dependent on the TFT characteristics, and especially the current threshold voltage of TFT has a great influence on the stability of the shift register, since the ununiformity or the drift of the current threshold voltage of TFTs constituting the shift register will cause malfunction or failure of the shift register.
However, at present, the manufacturing process for the low-temperature poly-silicon Thin Film Transistor is still in an immature stage, the current threshold voltage of the manufactured LTPS-TFT cannot be ensured consistency effectively and thus there is inconsistence among the current threshold voltages of respective LTPS-TFTs. Meanwhile, crystallization of amorphous silicon film is required for the LTPS technology, which may cause difference among the current threshold voltages of LTPS-TFTs in the glass substrate, particularly in the curve of the source-drain current and the gate-source voltage (Ids-Vgs) characteristics, the (Ids-Vgs) will drift as the current threshold voltage drifts. As for an amorphous silicon TFT, after a long-term operation, its current threshold voltage will drift under the influence of a positive stress, and particularly in the curve of the source-drain current and the gate-source voltage (Ids-Vgs) characteristics, the (Ids-Vgs) will drift as the current threshold voltage drifts.
Below, reasons for the failure of a shift register will be explained by taking the Ids-Vgs transfer characteristics curves of an N-type TFT manufactured by the LIPS technology in a normal state and in an abnormal state as shown in FIGS. 1 and 2, respectively as an example. The curve represented by a reference numeral “a” is obtained when the source-drain voltage is 10.1 V, and reflects the transfer characteristics when the TFT is in a saturation state. The curve represented by a reference numeral “b” is obtained when the source-drain voltage is 0.1 V, and reflects the transfer characteristics when the TFT is in a non-saturation state. It can be seen from FIG. 1 that, in the non-saturation state, when the gate-source voltage is 0V, the off-state current (that is, the source-drain current) is smaller than 10−11 A, but the off-state current is greater than 10−9 A when the gate-source voltage is 0V in FIG. 2. In FIG. 2, the TFT operates in a sub-threshold operation region wherein there is an exponential relationship between the source-drain current and the gate-source voltage, the off-state current will varies significantly when the gate voltage has a certain disturbance. The shift register constituted by the TFTs with the characteristics curve in FIG. 2 has the following two problems: first, a large off-state current will cause increase in the power consumption of the shift register; secondly, a too large off-state current will result in that it is difficult to maintain the charges on the storage capacitor in the shift register, and thus the thin film transistor associated cannot be turned off normally and in turn the shift register cannot operate normally.
In summary, the phenomenon that the current threshold voltage of a thin film transistor drifts may seriously affect the normal turn-off of the thin film transistor, and in turn affects the normal operation of the circuit configuration including the thin film transistor; in the shift register, if the current threshold voltage of the thin-film transistor which controls the signal output is inconsistent or drifts, the normal output of the entire shift register is affected directly.